[mono][wasm] marshal-ilgen is dropped when not required (#86035)

[mono][wasm] marshal-ilgen is dropped when not required (#86035)

* Replicating naricc's PInvokeScanner.

* MarshalingPInvokeScanner now detects and outputs incompatible assemblies.

* Allowing void return type.

* PInvoke

* Two-pass searching in progress.

* Second pass resolves inconclusive types.

* Cleanup.

* Modifying the wasm toolchain to omit marshal-ilgen when possible.

* Hopefully fix incorrect app dir.

* Added definitions to MarshalingPInvokeScannerPath hopefully where needed.

* Adding definitions of MarshalingPInvokeScannerPath to more locations.

* Adding missing references to PInvoke scanner.

* Changed task ordering, assemblies list.

* Removed metadata load context.

* Fixed code analyzer issues.

* Fixed file name.

* Moved MarshalingPInvokeScanner to MonoTargetsTask.

* Removed BlazorApp.

* Implemented more marshaling validation rules, removed warning message that got "promoted" to an error.

* Catching bad image exceptions, giving reason for requiring marshal-ilgen.

* Cleaned up references to standalone MarshalingPInvokeScanner project, now that the analyzer is in MonoTargtesTask.

* More cleanup.

* Fixed P/Invoke return value in marshal-lightweight.

* Removed incompatible assemblies listing.

* Restoring minimal functionality to marshal-ilgen-stub.

* Addressed feedback.

* Tweaked identification of blittable types. Added explanation to Compatibility enum.

* Moved PInvokeCollector.cs and SignatureMapper.cs back to WasmAppBuilder.

* Addressed feedback.